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these, the discharge eventually went on to exceed the critical threshold dischar~Je of 30,000 C.fll. In the year when the peak discharge was greater than 18,00'0 cts but less than 30,000 cfs (1973), a lot of ~Iater still overtopped Highway 55 so closing the highway struct'ure is somethin9 t:hat would not have appeared to be unreasonable. The HEC-1 results, as noted in Appendix A, indicated longer flood warning times than what were calculated using an average velocity approach and the rule of thumb that flood waves in natural channels move about 1.5 times faster than the average flow velocity. The HEC-l resultll in Table 4 show flood warning times of 13.5 to 20. Cl hours, whereas the average velocity approach, as noted in the footnote to Table 1, indicates flood w.lrning tilnes of 10 hours or less. The HEC-l results may accurately represent the way flood waves move down the South Platte River, but for now it is recommended that the Crook flood warning program rely on having only 10 hours of warning time from when the high .llarm is sounded in Sterling. Experience gained by actually 'Jperating the system over the years will help to tell if longer flood warning times are available, as indil::ated by the HIEC:-]. results in Table 4. 3.3 Flood Warning Options Two options for the configuration of the South Platte River at the Crook Flood Warning System have been discussed by the Omaha Distric:t and the CWCB. They are summarized as follows as Options 1 and 2. These options will be further discussed in Part '7, "Comm,ants .'lrld Recommendations". 3.3.1 Option 1 This option consists of an automated flood detection device roughly 130 feet downstr,a.lm of the Highway 6 Bridge at Sterling and a staff gage downstre,am of t:he Highway 55 crossing at Crook. An observer would have to be designated to read the Crook staff gage d1.1ring flO'Jds. 3.3.2 Option 2 This option consists c.f staff gages at both Sterling and Crook. Volunteer observers would haVEl to be found at both towns to read the Istaff gagcas dud.n91 floods. 4. FLOOD WARNING EQUIPMEWr AND INSTALLATION 4.1 Staff Gages 4.1.1 General Inf'Jrmation Staff gages are the oldest and most basic method of keeping track of river stag,as. st:aff gages can either be made or purchased from vendors of strcaam gaging equipment. There is a considerable cos1l:. savings if YCl\l1 build your own. The Omaha District makes its own staff gages out of treated 2" x 8" lumber. A table saw :Ls used to cut grooves in the lumber to l:Jraduate the llc:ale in feet and tenths of a 9